X Video Extension
Protocol Description
Version 2
25-JUL-91
David Carver
Digital Equipment Corporation
Workstation Engineering/Project Athena
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Copyright 1991 by Digital Equipment Corporation, Maynard, Massachusetts,
and the Massachusetts Institute of Technology, Cambridge, Massachusetts.
All Rights Reserved
Permission to use, copy, modify, and distribute this software and its
documentation for any purpose and without fee is hereby granted, provided
that the above copyright notice appear in all copies and that both that
copyright notice and this permission notice appear in supporting
documentation, and that the names of Digital or MIT not be used in
advertising or publicity pertaining to distribution of the software
without specific, written prior permission.
DIGITAL DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE, INCLUDING
ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS, IN NO EVENT SHALL
DIGITAL BE LIABLE FOR ANY SPECIAL, INDIRECT OR CONSEQUENTIAL DAMAGES OR
ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER
IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING
OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
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Preface
-------
The following is an outline for an X video extension protocol. It
is preliminary and subject to change. My goal in writing this was
to fix some the shortcomings of existing overly simplistic
extensions while avoiding pitfalls in an overly complex extension.
Your feedback is desired, and since the major design directions
have been stable for some time, feel free to hammer on the details
of the protocol.
When you receive a revision of the document, refer to the changes
and issues sections to guide your review and analysis.
Acknowledgements
---------------
The following people have made major contributions to the design of
the Xv protocol:
Branko Gerovac (DEC/Corporate Research)
Russ Sasnett (GTE/Project Athena)
Ralph Swick (DEC/Project Athena)
Many ideas and approaches in Xv were the product of discussions
with several groups, including
Project Athena's Visual Computing Group
The MIT X Consortium
The MIT Media Lab's Interactive Cinema Group
Changes
-------
From version 1.3 to 2.0
-- Changed SetPortControl and GetPortControl to GetPortAttribute
and SetPortAttribute.
-- Changed QueryBestSize
-- Simplified SelectVideoNotify and SelectPortNotify requests.
-- Changed the way SetPortControl and GetPortControl works.
-- Added a QueryExtension request to return the version and
revision information of the extension.
-- Changed the name of the QueryVideo request to QueryAdaptors;
Removed the list of encodings from QueryVideo and added a
QueryEncodings request.
-- Added a PortNotify event that notifies interested clients that
a port control has been changed.
-- Added SelectPortNotify request to select for PortNotify events.
-- The XvInterruped reason has been replaced by two new reasons:
one for when video is preempted by another video request and
one for when video is terminated because of hard transmission
or reception errors.
-- Changed the wording of the QueryBestSize request. Added issue
about whether or not returned sizes should maintain the
requested aspect ratio.
Introduction
------------
Video technology is moving very quickly. Standards for processing
high resolution video are currently a hot topic of discussion
internationally, and it will soon be possible to process video
entirely within the digital domain. The Xv extension, however,
does not attempt to address issues of digital video. Its purpose
is to provide a mechanism for support of current and near term
interactive video technology.
It is somewhat ironic that Xv contains nothing particularly
innovative. It takes a minimalistic approach, and without a doubt
it could have been defined years ago, and with several revisions.
So, the life expectancy of Xv is not long. Nevertheless, it may
undergo further revision and experimentation that will help our
progress towards digital video systems.
One premise of the Xv extension is that the X server is not alone.
A separate video server is often used to manage other aspects of
video processing, though the partition between what the X server
does and what a video server does is a matter of great debate.
Model
-----
This extension models video monitor capabilities in the X Window
System. Some advanced monitors support the simultaneous display
of multiple video signals (into separate windows), and that is
prepresented here through the ability to display video from
multiple video input adaptors into X drawables.
Some monitors support multiple video encodings (mostly for
internationalization purposes) either through switches or
automatic detection, thus each video adaptor specifies the set of
encodings it supports.
The requests to display video from an adaptor into a drawable are
modeled after the core PutImage request, though extended to
support scaling and source clipping.
Video output is also supported and is symmetric with the video
input function, though fewer GC components are used.
Mechanism
---------
The Xv extension does the following:
-- lists available video adaptors
-- identifies the number of ports each adaptor supports
-- describes what drawable formats each adaptor supports
-- describes what video encodings each adaptor supports
-- displays video from a port to a drawable
-- captures video from a drawable to a port
-- grabs and ungrabs ports
-- sets and gets port attributes
-- delivers event notification
Adaptors
--------
A display may have multiple video input and output adaptors. An
adaptor may support multiple simultaneously active ports, and in
some cases the number of ports has no fixed limit.
An input port receives encoded video data and converts it to a
stream of data used to update a drawable. An output port samples
data from a drawable and produces a stream of encoded video data.
The ADAPTORINFO structure is used to describe a video adaptor.
ADAPTORINFO:
[base-id: PORT
num-ports: CARD16
type: SETofADAPTORTYPE
formats: LISTofFORMAT
name: STRING]
ADAPTORTYPE: {Input, Output}
FORMAT:
[depth: CARD8
visual: VISUALID]
The base-id field specifies the XID of the first port of the
adaptor. The `num-ports' field specifies how many ports the
adaptor supports. The ports of the adaptor have XIDs in the range
[base-id..base-id + num-ports - 1]
The type attribute determines if the adaptor can process video
input, output, or input and output. The if the adaptor can
process input then Input is asserted, if the adaptor can process
output then Output is asserted.
The drawable depths and visual types supported by the adaptor are
listed in `formats'. Note: that when video is being processed for
pixmaps the visual format is taken to be the visual of the first
pair that matches the depth of the pixmap.
The name field contains an a vendor specific string that
identifies the adaptor.
It should be noted that the existence of separate adaptors doesn't
necessarily imply that simultaneous operation is supported.
Errors
------
Port
A Port error is returned if any request names a PORT that does not
exist.
Encoding
An Encoding error is returned if any request names an ENCODINGID
that does not exist.
Query Requests
-------------------
QueryExtension
==>
version: CARD16
revision: CARD16
The QueryExtension request returns the extension version and
revision numbers.
QueryAdaptors
win: WINDOW
==>
adaptors: LISTofADAPTORINFO
The QueryAdaptors request returns the video adaptor information for
the screen of the specified window.
Errors: {Window}
QueryEncodings
port: PORT
==>
encodings: LISTofENCODINGINFO
The QueryEncodings request returns the list of encodings supported
by the port adaptor. Use the SetPortAttribute request to set
which encoding a port is to process. The ENCODINGINFO record
describes an encoding:
ENCODINGINFO:
[encoding: ENCODINGID
name: STRING
width, height: CARD16
rate: FRACTION]
The `encoding' field identifies an encoding supported by a port.
Its value is unique for a screen. Width and height specify the
size of the video image and rate specifies the rate at which
fields of image information are encoded.
An encoding is identified by a string that names the encoding.
Encoding naming conventions need to be established (i.e.,
something along the lines of font naming, but simpler)
FRACTION
[numerator, denominator: INT32]
The FRACTION structure is used to specify a fractional number.
Errors: {Port}
Put Video Requests
------------------
PutVideo
port: PORT
drawable: DRAWABLE
gc: GCONTEXT
vid-x, vid-y: INT16
vid-w, vid-h: CARD16
drw-x, drw-y: INT16
drw-w, drw-h: CARD16
The PutVideo request writes video into a drawable. The position
and size of the source rectangle is specified by vid-x, vid-y,
vid-w, and vid-h. The position and size of the destination
rectangle is specified by drw-x, drw-y, drw-w, drw-h.
Video data is clipped to the bounds of the video encoding, scaled
to the requested drawable region size (or the closest size
supported), and clipped to the bounds of the drawable.
If video is successfully initiated, a VideoNotify event with
detail Started is generated for the drawable. If the port is
already in use, its video is preempted, and if the new drawable is
different than the old, a VideoNotify event with detail Preempted
is generated for the old drawable. If the port is grabbed by
another client, this request is ignored, and a VideoNotify event
with detail Busy is generated for the drawable. If the port is
not receiving a valid video signal or if the video signal is
interrupted while video is active a VideoNotify event with detail
HardError is generated for the drawable.
GC components: subwindow-mode, clip-x-origin, clip-y-origin, clip-mask.
Errors: {Match, Value, GContext, Port, Alloc}
PutStill
port: PORT
drawable: DRAWABLE
gc: GCONTEXT
vid-x, vid-y: INT16
vid-w, vid-h: CARD16
drw-x, drw-y: INT16
drw-w, drw-h: CARD16
The PutStill request writes a single frame of video into a
drawable. The position and size of the source rectangle is
specified by vid-x, vid-y, vid-w, and vid-h. The position and
size of the destination rectangle is specified by drw-x, drw-y,
drw-w, drw-h.
Video data is clipped to the bounds of the video encoding, scaled
to the requested drawable region size (or the closest size
supported) and clipped to the bounds of the drawable.
If the port is grabbed by another client, this request is ignored,
and a VideoNotify event with detail Busy is generated for the
drawable. If the port is not receiving a valid video signal a
VideoNotify event with detail HardError is generated for the
drawable.
GC components: subwindow-mode, clip-x-origin, clip-y-origin, clip-mask.
Errors: {Match, Value, GContext, Port, Alloc}
Get Video Requests
------------------
GetVideo
port: PORT
drawable: DRAWABLE
gc: GCONTEXT
vid-x, vid-y: INT16
vid-w, vid-h: CARD16
drw-x, drw-y: INT16
drw-w, drw-h: CARD16
The GetVideo request outputs video from a drawable. The position
and size of the destination rectangle is specified by vid-x,
vid-y, vid-w, and vid-h. The position and size of the source
rectangle is specified by drw-x, drw-y, drw-w, and drw-h.
Drawable data is clipped to the bounds of the drawable, scaled to
the requested video region size (or the closest size supported)
and clipped to the bounds of the video encoding. The contents of
any region not updated with drawable data is undefined.
If video is successfully initiated, a VideoNotify event with
detail Started is generated for the drawable. If the port is
already in use, its video is preempted, and if the new drawable is
different than the old, a VideoNotify event with detail Preempted
is generated for the old drawable. If the port is grabbed by
another client, this request is ignored, and a VideoNotify event
with detail Busy is generated for the drawable.
GC components: subwindow-mode, clip-x-origin, clip-y-origin,
clip-mask.
Errors: {Match, Value, GContext, Port, Alloc}
GetStill
port: PORT
drawable: DRAWABLE
gc: GCONTEXT
vid-x, vid-y: INT16
vid-w, vid-h: CARD16
drw-x, drw-y: INT16
drw-w, drw-h: CARD16
The GetStill request outputs video from a drawable. The position
and size of the destination rectangle is specified by vid-x,
vid-y, vid-w, and vid-h. The position and size of the source
rectangle is specified by drw-x, drw-y, drw-w, and drw-h.
Drawable data is clipped to the bounds of the drawable, scaled to
the requested video region size (or the closest size supported)
and clipped to the bounds of the video encoding. The contents of
any region not updated with drawable data is undefined.
If the still is successfully captured a VideoNotify event with
detail Still is generated for the drawable. If the port is
grabbed by another client, this request is ignored, and a
VideoNotify event with detail Busy is generated for the drawable.
GC components: subwindow-mode, clip-x-origin, clip-y-origin,
clip-mask.
Errors: {Match, Value, GContext, Port, Alloc}
Grab Requests
-------------
GrabPort
port: PORT
timestamp: {TIMESTAMP, CurrentTime}
==>
status: {Success, AlreadyGrabbed, InvalidTime}
The GrabPort request grabs a port. While a port is grabbed, only
video requests from the grabbing client are permitted.
If timestamp specifies a time older than the current port time, a
status of InvalidTime is returned. If the port is already grabbed
by another client, a status of AlreadyGrabbed is returned.
Otherwise a status of Success is returned. The port time is
updated when the following requests are processed: GrabPort,
UngrabPort, PutVideo, PutStill, GetVideo, GetStill
If the port is actively processing video for another client, the
video is preempted, and an VideoNotify event with detail Preempted
is generated for its drawable.
Errors: {Port}
UngrabPort
port: PORT
timestamp: {TIMESTAMP, CurrentTime}
The UngrabPort request ungrabs a port. If timestamp specifies a
time before the last connection request time of this port, the
request is ignored.
Errors: {Port}
Other Requests
--------------
StopVideo
port: PORT
drawable: DRAWABLE
The StopVideo request stops active video for the specified port
and drawable. If the port isn't processing video, or if it is
processing video in a different drawable, the request is ignored.
When video is stopped a VideoNotify event with detail Stopped is
generated for the associated drawable.
Errors: {Drawable, Port}
SelectVideoNotify
drawable: DRAWABLE
onoff: BOOL
The SelectVideoNotify request enables or disables VideoNotify
event delivery to the requesting client. VideoNotify events are
generated when video starts and stops.
Errors: {Drawable}
SelectPortNotify
port: PORT
onoff: BOOL
The SelectPortNotify request enables or disables PortNotify event
delivery to the requesting client. PortNotify events are
generated when port attributes are changed using SetPortAttribute.
Errors: {Port}
QueryBestSize
port: PORT
motion: BOOL
vid-w, vid-h: CARD16
drw-w, drw-h: CARD16
==>
actual-width, actual-height: CARD16
The QueryBestSize request returns, for the given source size and
desired destination size, the closest destination size that the
port adaptor supports. The returned size will be equal
or smaller than the requested size if one is supported. If motion
is True then the requested size is intended for use with full
motion video. If motion is False, the requested size is intended
for use with stills only.
The retuned size is also chosen to maintain the requested aspect ratio
if possible.
Errors: {Port}
SetPortAttribute
port: PORT
attribute: ATOM
value: INT32
The SetPortAttribute request sets the value of a port attribute.
The port attribute is identified by the attribute atom. The
following strings are guaranteed to generate valid atoms using the
InternAtom request.
String Type
-----------------------------------------------------------------
"XV_ENCODING" ENCODINGID
"XV_HUE" [-1000..1000]
"XV_SATURATION" [-1000..1000]
"XV_BRIGHTNESS" [-1000..1000]
"XV_CONTRAST" [-1000..1000]
If the given attribute doesn't match an attribute supported by the
port adaptor a Match error is generated. The supplied encoding
must be one of the encodings listed for the adaptor, otherwise an
Encoding error is generated.
If the adaptor doesn't support the exact hue, saturation,
brightness, and contrast levels supplied, the closest levels
supported are assumed. The GetPortAttribute request can be used
to query the resulting levels.
When a SetPortAttribute request is processed a PortNotify event is
generated for all clients that have requested port change
notification using SelectPortNotify.
Errors: {Port, Match, Value}
GetPortAttribute
port: PORT
attribute: ATOM
==>
value: INT32
The GetPortAttribute request returns the current value of the
attribute identified by the given atom. If the given atom
doesn't match an attribute supported by the adaptor a Match
error is generated.
Errors: {Port, Match}
Events
------
VideoNotify
drawable: DRAWABLE
port: PORT
reason: REASON
time: TIMESTAMP
REASON: {Started, Still, Stopped, Busy, Preempted, HardError}
A VideoNotify event is generated when video activity is started,
stopped, or unable to proceed in a drawable.
A Started reason is generated when video starts in a drawable.
A Stopped reason is generated when video is stopped in a
drawable upon request.
A Busy reason is generated when a put or get request cannot
proceed because the port is grabbed by another client.
A Preempted reason is generated when video is stopped by a
conflicting request.
A HardError reason is generated when the video port cannot
initiate or continue processing a video request because of an
underlying transmission or reception error.
PortNotify
port: PORT
attribute: ATOM
value: INT32
time: TIMESTAMP
The PortNotify event is generated when a SetPortAttribute request
is processed. The event is delivered to all clients that have
performed a SelectPortNotify request for the port. The event
contains the atom identifying the attribute that changed, and the
new value of that attribute.